1. Field of Invention
The present invention relates to a photodiode which can be fabricated using standard semiconductor fabrication techniques.
2. Description of the Related Art
Infrared wireless data communication is a useful method for short range (in the approximate range of 0-10 meters) wireless transfer of data between electronic equipment; such as, cellular phones, computers, computer peripherals (printers, modems, keyboards, cursor control devices, etc.), electronic keys, electronic ID devices, and network equipment. Infrared wireless communication devices typically have the advantages of smaller size, lower cost, fewer regulatory requirements, and a well defined transmission coverage area as compared to radio frequency wireless technology (i.e. the zone of transmission is bounded by physical walls and therefore more useful in an office environment). In addition, infrared wireless communication has further advantages with regard to reliability, electromagnetic compatibility, multiplexing capability, easier mechanical design, and convenience to the user as compared to cable based communication technology. As a result, infrared data communication devices are useful for replacing 0-10 meter long data transfer cables between electronic devices, provided that their size and costs can be reduced to that of comparable cable technology. As examples of the type of wireless communications links that are presently in use, the Infrared Data Association (IrDA) Physical Layer Link Specification 1.1e specifies two main physical layer infrared modulation protocols.
The IrDA Physical Layer Link Specification 1.1e also specifies two modes for modulation of data on the infrared transmitted signal. One mode is a low-speed (2 Kbp/s to 1.15 Mbp/s) on-off infrared carrier using synchronous modulation where the presence of a pulse indicates a 0 bit and the absence of a pulse indicates a 1 bit. The second mode is a high speed (4 Mb/s) synchronous Four Pulse Position Modulation (4 PPM) method in which the time position of a 125 nS infrared pulse in a 500 nS frame encodes two bits of information. The 1.1e specification also specifies a preamble pattern which is sixteen repeated transmissions of a predetermined set of symbols.
Infrared data communications devices typically consist of transmitter and receiver components. The inflared data transmitter section consists of one or more infrared light emitting diodes (LEDs), an infrared lens, and an LED current driver. A conventional infrared data receiver typically consists of an infrared photodiode and a high gain receiver amplifier with various signal processing functions, such as automatic gain control (AGC), background current cancelling, filtering, and demodulation. For one-directional data transfer, only a transmitter at the originating end and a receiver at the answering end is required. For bi-directional communication, a receiver and transmitter at each end is required. A combined transmitter and receiver is called a transceiver.
A representative example of a conventional infrared data transmitter and receiver pair is shown in FIG. 1A. Infirared transmitter 10 includes LED 16 which generates a modulated infrared pulse in response to transistor 14 being driven by the data signal input at D.sub.IR. The modulated infrared signal is optically coupled to an infrared detector, such as photodiode 24 normally operated in current mode (versus voltage mode) producing an output current which is a linear analog of the optical infrared signal falling on it. The infrared pulses generated by LED 16 strike photodiode 24 causing it to conduct current responsive to the data signal input at D.sub.IR thereby generating a data signal received at D.sub.IR.
In receiver 20, the signal received at D.sub.IR is transformed into a voltage signal V.sub.IR and amplified by amplifier 26. The signal output from amplifier 26 then feeds into comparator 42 which demodulates the received signal by comparing it to a detection threshold voltage V.sub.DET in order to produce a digital output data signal at D.sub.OUT.
The received signal waveform will have edges with slope and will often include a superimposed noise signal. As a result, V.sub.DET is ideally placed at the center of the received signal waveform so that the output data signal has a consistent waveform width despite the slope of the received signal edges. Also, placing V.sub.DET at the center of the received signal improves the noise immunity of receiver 20 because the voltage difference between V.sub.DET and both the high and low levels of the received signal is maximized such that noise peaks are less likely to result in spurious transitions in D.sub.OUT.
The received signal, however, can vary in amplitude by several orders of magnitude due primarily to variations in the distance between transmitter 10 and receiver 20. The strength of the received signal decreases proportional to the square of the distance. Depending on the range and intensity of the infrared transmitter, the photodiode outputs signal current in the range of 5 nA to 5 mA plus DC and AC currents arising from ambient infrared sources of sunlight, incandescent and fluorescent lighting. As a consequence, the center of the received signal waveform will vary, whereas V.sub.DET must generally be maintained at a constant level. To address this problem, receivers typically include an automatic gain control (AGC) mechanism to adjust the gain responsive to the received signal amplitude. The received signal is fed to AGC peak detector 36 which amplifies the signal and drives current through diode 32 into capacitor 28 when the signal exceeds the AGC threshold voltage V.sub.AGC in order to generate a gain control signal. The gain control signal increases in response to increasing signal strength and correspondingly reduces the gain of amplifier 26 so that the amplitude of the received signal at the output of amplifier 26 remains relatively constant despite variations in received signal strength.
At a minimum, infrared receiver 20 amplifies the photodetector signal current and then level detects or demodulates the signal when it rises above the detect threshold V.sub.DET thereby producing a digital output pulse at D.sub.OUT. For improved performance, the receiver may also perform the added functions of blocking or correcting DC and low frequency AC ambient (1-300 uA) signals and Automatic Gain Control (AGC) which improves both noise immunity and minimizes output pulse width variation with signal strength.
The structure of the conventional discrete PIN photodiode 24 is illustrated in FIG. 1B. A wafer 50 is lightly doped with N dopant in order to produce an intrinsic region 56. A P+ region 52 is formed on one surface of the wafer and an N+ region 58 is formed on the opposing surface of wafer 50 with intrinsic region 56 interposed P+ region 52 and N+ region 58. A reflective layer 60, typically gold, is disposed on the surface containing P+ region 58 with reflective layer 60 also serving as the electrical contact to N+ region 58. A metal contact 54 is disposed on the surface containing P+ region 52 to provide the electrical connection to the P+ region.
Typically, one power supply potential is applied to the reflective layer 60 and another power supply voltage is applied to contact 54 to reverse bias the PN junction formed by P+ region 52 and N+ region 18. This forms a large depletion region within the intrinsic region 56 wherein electron and hole charge carrier pairs generated by light photons incident upon the intrinsic region 56 are rapidly accelerated toward the P+ and N+ regions respectively by the electric field of the reverse bias voltage. Charge carrier pairs are also typically generated outside the depletion region within intrinsic region 56 which diffuse, due to random thermal motion of the carriers, at a much slower velocity until they reach either the depletion region or the junction formed by P+ region 52 and intrinsic region 56 of photodiode 24.
A conventional photodiode that is designed for high quantum, i.e. light conversion, efficiency requires that the light path within the photo current collection zone, i.e. the depletion and non-depletion zones within intrinsic region 56, be sufficient in length so that most of the light photons of the incident light signal area are absorbed and converted into electron-hole pairs that are collectable at the P+ and N+ regions. Usually, this requires that the width of the intrinsic region 56, which is the primary light collection region, be several times the length required for light absorption. If diode 10 has an efficient back-side reflector, such as reflective layer 60, which effectively doubles the light path within diode 24, then the intrinsic region 56 of the photodiode can be made narrower. For a typical near infrared silicon photodiode, the nominal absorption path length is about 15-25 microns. The path length should be at least two to three times the nominal absorption path length to obtain good light conversion efficiency.
On the other hand, a photodiode designed for high frequency response requires that the photo current pairs generated by the light signal be collected rapidly and that the diode RC time constant is fast. Rapid photo current pair collection usually requires that most of the photo current pairs generated by the light signal be generated with the depletion region formed by the reverse bias voltage because the pairs will have a high drift velocity. Otherwise, the photo generated charge carrier pairs produced in the non-depletion regions within intrinsic region 56 and within diffusion distance of the collection electrodes 52 and 58 will have a diffusion velocity that is several hundred times slower than the velocity of the pairs generated within the depletion zone. The photo generated charge carrier pairs in the non-depletion zones will slowly migrate for collection at P+ region 52 and N+ region 58 resulting in a tail on the trailing edge of the electrical signal corresponding to the light signal. The diffusion distance of the charge carriers is determined by the carrier mean free path before re-combination and may exceed 150 microns.
A fast RC time constant for photodiode 24 requires minimal capacitance and low series resistance between the electrical contacts 54 and 60 and the photo current pair collection sites at the margin between P+ region 52 and the depletion zone and the margin between N+ region 58 and the depletion zone. The greater the width of the intrinsic region 56, the greater the width of the depletion zone and the lower the capacitance per unit area of photodiode 24. Since the width of the depletion zone increases with the magnitude of the reverse bias voltage, it is typical for high speed photodiodes to have a relatively high reverse voltage applied to them.
The inclusion of lightly doped intrinsic region 56 between the P+ and N+ regions 52 and 58 results in a PIN photodiode with a wider depletion region, depending on the magnitude of the reverse bias voltage, which improves the light collection efficiency, increases speed, and reduces capacitance over that of a simple PN diode structure.
The PIN photodiode is typically produced by diffusing the N+ region 58 on the back side of the lightly doped (N) wafer 50, diffusing the P+ region 52 on the topside of the wafer 50, and then adding metal contacts to each side of the wafer. Typically, the backside contact area connected to N+ region 58 is reflective layer 60 and is made of gold. The reflective layer is then typically connected to the ground voltage terminal.
A PN diode junction can also function as a photodiode. However, the photo-current collection region within an electric field, the drift region, in a PN photodiode is limited to the relatively thin depletion zone produced when the PN junction is reverse biased. This thin drift region is much less efficient in the collection of photo-generated charge carrier pairs because most of the pairs are generated outside of depletion zone. Also, the charge pairs generated outside of the depletion zone thermally diffuse to collection points margins of the P and N layers and into the depletion zone at a much slower relative speed resulting in slow photodiode performance. In addition, the highly doped P and N regions result in high diode capacitance per unit area which further slows the performance of the photodiode.
Although a PIN photodiode outperforms a standard PN diode, the PIN photodiode structure cannot be easily manufactured by standard semiconductor processes wherein fabrication is typically performed on only one side of the semiconductor wafer 50.
In typical high volume applications, it is now, standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. As described above, it is difficult to integrate an efficient photodiode on the same semiconductor substrate as the transceiver circuit. As a result, a discrete infrared photodiode is typically assembled with the transceiver circuit and an LED, along with lenses for the photodiode and LED, into a plastic molded package to form a transceiver module. The transceiver module is designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view, typically through an infrared window on the transceiver casing. The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.